All 4 cylinders held 150psi with no bubbles through to the cooling system indicating head gasket ok. Changed the spark plugs to Denso IK27s and car now runs well but still get bubbling in header tank after switching off. Will keep eye on it and get it ran in. Cant believe spark plugs could make such a difference but I had incorrectly fitted QC59. Thanks for the advice to change to these from Motorsport Developments. I've got a new thermostat to fit yet. I'm not convinced I'm out of the woods yet but a lot happier than yesterday.